Take this image of Jack-Jack from the Incredibles, for example.
I really wanted to practice my face-shading skills using this image. So I imported the image into Photoshop and lowered the opacity, then proceeded to trace over the image on another layer. My results?
Not pretty, I know. But keep in mind that I am creating an outline solely for the purpose of coloring it in. Any outlines are later going to be replaced by color and I have added some extra lines to mark important transitions in shading.
This step can be done in a variety of ways, depending on what the purpose of your final art piece will be. If you want it to be a simple drawing or some line art, then focus on only the major features...don't outline everything like I did. Make sure the lines are much smoother than I made them.
The next step would be to transition the drawing to paper that you intend your final piece to be on. Now, if you've created some line art or a drawing within photoshop, you can just print it out on your desired paper. But if you've created an outline that's not going to part of your final piece, like I did, you're going to want to have this printed on your paper with pencil.
How can you do this? Start by printing out the work you've done in photoshop and color over the back with graphite pencil. Make sure you've laid down a dark layer to ensure that the color will be picked up when you transfer your image to your desired paper.
Following this, place your printed outline with the graphite face down on your desired paper. Simply trace over the printed outline with your pencil, and the graphite will transfer from the back of your printed outline to your final paper.
As an alternative, you could lower the opacity of your outline so that it's really light. As long as you're working with fairly dark colors, it should be fairly easy to cover up the printed outline.
Now that you have your outline or line art on paper, go crazy with the coloring! Let me know how this works out for you in the comments below.